His rivals descended this week on what is essentially his home turf and they are ready to challenge the former Massachusetts governor on the economy, his central campaign theme, in a Tuesday night debate focused on that issue.

But don’t expect the criticism from Romney’s opponents to stop there, given that time is quickly running out before the first votes are cast in the GOP nominating fight.

“Even the richest man can’t buy back his past,” Texas Gov. Rick Perry’s campaign said in a web video that describes Romney as the inspiration for President Barack Obama‘s national health care overhaul that conservatives detest and questions Romney’s assertion that he is a “conservative businessman.”

Rep. Michele Bachmann of Minnesota on Monday urged conservative voters not to support for a candidate who isn’t one of them. “It’s not good enough to settle for anyone but Barack Obama,” she said while campaigning in New Hampshire.

Even lower-profile rivals are trying to knock Romney off his game.

“Simply advocating more ships, more troops and more weapons is not a viable path forward,” former Utah Gov. Jon Huntsman said Monday in a foreign policy speech that was a direct rebuttal to Romney’s a week earlier.

Romney, a second-time presidential contender, has a comfortable lead in New Hampshire polling after virtually camping out in the state since his 2008 loss and building a strong statewide campaign network. To stop him, his opponents have readied criticism on health care policy, cultural issues and environmental positions.

Tuesday’s debate was designed to be on the economy – voters’ top concern in a nation that recorded 9.1 percent unemployment last month – but there was scant chance Romney would be able to dodge questions about his record.

“For some people in campaigns their process is one of obfuscation and bewilderment,” Romney said while visiting a VFW hall. “You’re going to find in a campaign like this people running against me who will take what I said and try to say something else. There’s an ad out there today that does that.”

Perry and Bachmann both have tried to pitch themselves as an alternative to Romney, who signed into law a health care mandate as Massachusetts governor that conservatives loathe.

However, the candidates stumbled in their early attempts: Perry flubbed a practiced criticism during his last debate; Bachmann struggled to hold onto her fast rise in popularity and has struggled to gain traction for her message that casts Romney as a moderate who can’t be trusted.

The criticism has not thrown Romney’s pace. Nothing, to this point, has started an exodus among his supporters. And time is running short for Romney’s rivals to make that happen.

While New Hampshire has yet to schedule its primary, it is likely to come before mid-January. That means there are fewer than 100 days for the newcomers to make inroads in New Hampshire, a state where Romney is well known, owns a vacation home and won a second-place finish in his 2008 presidential bid.

Yet, his rivals note, Romney hasn’t faced steady nasty attacks here on television. His chief opponent four years ago, Sen. John McCain, didn’t have the campaign cash to buy the commercials.

That is not the case now.

Perry is sitting on $15 million. Bachmann has yet to report her fundraising, but previous campaigns show she is a prodigious fundraiser who isn’t shy to spend. And Huntsman, a former Utah governor who has made New Hampshire his make-or-break state, has a personal fortune he could tap; his allies have established an independent organization that could run anti-Romney ads with their almost unmatched family fortune.

Perry’s campaign signaled their anti-Romney ads were almost certain to start soon.

At the ready: a strong defense of Romney, emphasizing his record as a business executive whose campaign has been based almost exclusively on the economy. Since coming up short four years ago, he and his advisers have laid the extensive groundwork to respond quickly with a message tailored to the economic uncertainty.

If voters’ decision comes down to the economy, Romney is ready to highlight his accomplishments – and his rivals’ shortcomings.